Detection apparatus



July 2, 1968 mDDLE ET AL DETECTION APPARATUS Origina l Filed Aug. 23, 1961 12 Sheets-Sheet 1 JOLXI 3551:

ARNDT B. BERGH CHARLES C. FORGE ATTOPAYS DETECTION APPARATUS Original Filed Aug. 23, 1961 12 Sheets-Sheet 2 INVENTORS JOHN B. RIDDLE ARNDT B. BERGH CHARLES O. FORGE ATTOPNEVS July-2,1968 J. a. mum; ETAL 3,391,388

DETECTION APPARATUS Original Filed Aug. 23, L961 12 Sheets-Sheet 5 INVENTORS JOHN B. RIDDLE ARNDT 8. BERGH CHARLES O. FORGE Br M 2;

A T TOPNEYS July 2, 1968 J. B. RIDDLE ET AL 3,391,388

12 Sheets-Sheet 4 Origin 8.]. Filed Aug. 23, 1961 ONV INVENTORS JOHN RIDDLE ARNDT B. ERGH CHARLES C. FORGE 8y @ba/I M ATTORNE S J. B. RIDDLE ET AL DETECTION APPARATUS July 2, 1968 12 Sheets-Sheet 5 Original Filed Aug. 23, 1961 INVENTORS JOHN B, RIDDLE A-RNDT B. BERGH CHARLES O. FORGE By 5- M A TTOPNEYS HHMHWH d2 RIDDLE BERGH IIgVENTORS July 2, 1968 J. a. RIDDLE ET AL DETECTION APPARATUS 12 Sheets-Sheet 6 Original Filed Aug. 23, 1961 JOHN ARNDT 5 CHARLES O. FORGE ZOmhOm Ih mohuuhmo ATTORNEYS y 1963 J. a. RIDDLE ET AL DETECT I ON APPARATUS 12 Sheets-Sheet 7 Original Filed Aug. 23, 1961 INVENTORS JOHN B. RIDDLE ARNDT B. BERGH CHARLES o. FORGE A T TOQNEYS y 1968 .1. B. RIDDLE ET AL DETECTION APPARATUS l2 Sheets-Sheet 8 Original Filed Aug. 25, 1961 I :but

Stuns 056 INVENTORS JOHN B. RIDDLE ARNDT B. BERGH CHARLES O. FORGE ATTORNEYS July 2, 1968 J. B. RIDDLE ET AL 3,391,388

DETECT ION APPARATUS Original Filed Aug. 23, 1961 12 Sheets-Sheet 9 Q 9' k a 0. E

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VARIABLE BIAS PHASE INVERTER JOHN B. RIDDLE ARNDT a. BERGH CHARLES o. FORGE A TTORNEYS i July 2,1968 4 J. B. RIDDLE ET AL 3,391,388

DETECTION A-PPARATUS Original Filed Aug. 23, 1961 12 Sheets-Sheet 10 INVENTORY JOHN B. RIDDLE ARNDT B BERGH CHARLES Q. FORGE 12 Sheets-Sheet 11 .W Q2 5 5.5.. 55E v 81 L r 02 5 ST v01 5ZwBQ m mohwwvwm maozomzuim @8538 u 8T 52:5 5? -25 July 2, 1968 J. B. RIDDLE ETAL DETECTION APPARATUS Original Filed Aug. 23, 1961 INVENTORS JOHN B. RIDDLE ARNDT B. BERGH CHARLES 0. FORGE By Z.yM L! ATTORNEYS y 2, 1963 J. B. RIDDLE ET AL 3,391,388

I DETECTION APfARATUS original Filed Aug. 23, 1961 12 Sheets-Sheet 12 INVHVTORS JOHN B. RIDDLE ARNDT B. BERGH CHARLES O. FORGE A T TO/PNEVS United States Patent 3,391,388 DETECTIQN APPARATUS John B. Riddle, 180 Waller Hays Drive, and Arndt B. Bergh, 36% Evergreen Drive, both of Palo Alto, Calif. 94303, and Qharles 0. Forge, 20691 Homestead Road, Cupertino, Calif. 95014 Original appiication Aug. 23, 1961, Ser. No. 149,799, new Patent No. 3,280,974, dated Oct. 25, 1966. Divided and this application July 25, 1966, Ser. No. 596,024

8 Claims. (Cl. 340-149) ABSTRACT OF THE DISCLQSURE A method for recognizing individual ones of a plurality of predetermined signals by subdividing the signals into different portions where the signals differ from each other as indicated in FIG. 11, reversing the polarity of an unknown signal during one group of the subdivisions of the unknown signal, and integrating the unknown signal.

This application is a division of our earlier filed application Ser. No. 149,799, filed Aug. 23, 1961, now Patent No. 3,280,974 issued Oct. 25, 1966.

This invention relates to apparatus for recognizing printed matter in which the printed matter has a predetermined pattern indicia on a face thereof.

It is a principal object of the invention to provide apparatus for identifying printed matter by sensing the pattern of printed indicia on pieces of such printed matter.

It is another object of the invention to provide such apparatus for identifying printed matter which is sensitive to the existence or non-existence of very small fragments of the pattern of indicia on printed matter.

It is another object of the invention to provide such apparatus for identifying printed matter which is sensitive to the existence of these small fragments of indicia and which recognizes pieces of printed matter responsive to recognition of a sequence of these fragments.

It is another object of the invention to provide such apparatus for identifying printed matter which employs a detection element movable with respect to pieces of printed matter and adapted to generate an electrical signal responsive to the existence or non-existence of such very small ragments of the pattern of indicia on the printed matter.

It is another object of the invention to provide such apparatus in which the electrical signal is generated by the detection element only when the small fragment of the pattern of indicia adjacent to the detection element is nonuniform, so that no signal will be generated by the detection element when it is moved with respect to a completely uniform piece of printed matter or a blank piece on which no printing appears.

it is another object of the invention to provide apparatus for identifying printed matter in which a detection element scans the pattern of indicia on a piece of printed matter and in which the detection element sees only the smallest possible fragment of the pattern of indicia at each instant while it is scanning the pattern whereby the detection element sees the pattern in maximum detail.

It is another object of the invention to provide apparatus for identifying the individual members of a group of pieces of printed matter in which every piece of printed matter in the group has a predetermined series of predetermined patterns of indicia printed thereon but where the distances between adjacent patterns on diiferent pieces of the group are different.

It is another object of the invention to provide such apparatus for identifying the members of the group of pieces of printed matter in which a detection element scans a piece of printed matter along a path which extends across the series of printed patterns thereon.

It is another object to provide apparatus for identifying the validity of pieces of printed currency of a given denomination by scanning the currency with a detector, such as a magnetic flux detecting head for detecting magnetized ink on the currency, an optical fiber for detecting illuminated areas of the printed pattern on the currency, a heat sensitive element for detecting heated ink on the currency, etc., and where a counterpart of a valid piece of currency of that denomination is provided with a pickup element movable with respect to the counterpart in synchronization with scanning of the currency by the detector, the detector being connected to the counterpart to initiate and restart movement of the pickup element with respect to the counterpart a plurality of times while the detector scans a given piece of the currency whereby the apparatus may recognize substantially all pieces of currency of the given denomination and reject all other printed matter even though the pieces of currency of the given denomination have a plurality of local areas of printed indicia thereon with each piece of currency of that denomination having all of the patterns but with the patterns spaced from each other by different distances on different pieces.

It is another object of the invention to provide apparains for identifying pieces of printed matter in which a detection element is moved with respect to the pattern of indicia on a piece of printed matter and generates an electrical detection signal which is a parameter of the pattern but where the detection signal bears the same parametric relation to the pattern of indicia regardless of the relative speed with which the detection element and pattern are moved with respect to each other.

It is another object of the invention to provide such apparatus for identifying pieces of printed matter which will be substantially completely automatic in operation so that it may be left unattended and may be operated simply by the delivery thereto of a piece of printed matter to be identified.

It is another object of the invention to provide such automatic apparatus which may be left unattended and which will contain components of minimum monetary value to the average person, whereby tampering with the apparatus may be avoided.

It is another object of the invention to provide apparatus for identifying pieces of printed matter in which pieces of printed matter to be identified are inserted into the apparatus and in which an identifiable piece of printed matter will be identified regardless of the manner in which the piece is inserted into the apparatus.

It is another object of the invention to provide apparatus for recognizing the similarity between a piece of printed matter being examined and a predetermined standard reference.

It is another object of the invention to provide such recognition apparatus in which any desired predetermined degree of similarity may be required between the standard reference and the piece of printed matter being examined before recognition will be made.

It is another object of the invention to provide such apparatus for identifying printed matter which is simple and economical to make and dependable and trouble free in use, yet which will be suificiently complex as to prevent all attempts to intentionally make it identify undesirable pieces of printed matter.

It is another object of the invention to provide apparatus for identifying printed matter which is sensitive to the quality of the ink with which a pattern of indicia is printed on such printed matter.

It is another object of the invention to provide apparatus of this type which is particularly well suited for identiiying printed currency.

It is another object of the invention to provide apparatus of the greatest reliability for recognizing the validity and denomination of pieces of printed currency.

It is another object of the invention to provide such apparatus for recognizing the validity and denomination of pieces of printed currency which will be able to recognize all valid bills of a given denomination regardless of any differences between them caused by excessive use or the fact that they were printed at different times from different plates.

It is another object of the invention to provide such apparatus for recognizing all valid bills of a given denomination but which will refuse to recognize invalid bills an valid bills of other denominations.

It is another object of the invention to provide such apparatus for recognizing currency which will recognize all of the bills of a plurality of denominations rejecting all other pieces of printed matter and which will distinguish between the bills of the different denominations which it recognizes.

It is another object of the invention to provide such apparatus for recognizing currency which will refuse to recognize fractions of valid prices of currency.

It is another objects of the invention to provide such apparatus for recognizing printed currency by comprising an unknown bill to a predetermined physical counterpart.

While it is contemplated that the physical counterpart may be a piece of currency, it is an object of the invention to provide such apparatus in which the counterpart to which an unknown bill is compared is a synthetic element, not itself a counterfeit, derived from a valid bill of known denomination and which will have sutficiently little monetary value that people will not attempt to remove it from the apparatus.

It is another object of the invention to provide apparatus for recognizing printed currency which apparatus may be operated automatically and needs very little care so that it can be installed and used in unattended places as a part of a change making machine, automatic vending machine or the like.

It is another object of the invention to provide a cur rency recognition device in which a bill may be inserted along a path parallel to the length of the bill and in which the apparatus can recognize a valid bill regardless of which end of the bill is inserted first and regardless of which side of the bill is facing upwardly when the bill is inserted.

It is another object of the invention to provide apparatus for recognizing printed currency having a detection head for scanning pieces of such currency to be identified and means for moving the piece of currency and head with respect to each other and comparing the area of the printed pattern on the currency adjacent to the head to a predetermined standard reference as the head and piece of currency are moved with respect to each other.

It is another object of the invention to rovide such apparatus for recognizing currency in which means are provided for resynchronizing comparison between the pattern on the bill and the reference standard a plurality of times while the head and bill are moving with respect to each other so that the apparatus can recognize as one type of bill a plurality of separate bills on which corresponding printed areas are differently placed.

It is another object of the invention to provide such apparatus for recognizing currency in which the detection head, at each instant during relative movement between bill and head, is sensitive to only a very small fragment of the pattern of indicia on the bill so that the head sees the printed pattern in sufiicient detail to detect the fact that the bill was printed from poorly engraved plates.

It is another object of the invention to provide apparatus for recognizing the validity and denomination of printed currency by magnetizing the ink on said currency and then analyzing the pattern of magnetic flux adjacent to the urinted indicia on the currency.

It is another object of the invention to provide such currency recognition apparatus in which means are provided for magnetizing the ink on bilis in a direction per pendicular to the face of the bill and then moving the face of the bill past a magnetic flux measuring device, or magnetometer, to produce a detection signal from the magnetometer which is a parameter of the flux sensed by the magnetometer while the bill moves with respect theret0.

As used herein the term magnetometer means a device for statically sensing magnetic flux in space and gencrating an electrical signal which is a parameter of the fiux sensed. The magnetometer contains a head portion for sensing static flux in a local area of space and a power means portion connected to the magnetometer for generating an electrical signal which is a parameter of the flux sensed by the head portion. The magnetometer is thus to be distinguished from what may be called a dynamic head which is a magnetic measuring device which gencrates an electrical signal as a parameter of the rate of movement of static magnetic flux adjacent to the device, the movement of flux resulting from the movement of the dynamic head in a static magnetic field, the movement of a magnetic field past a stationary dynamic head, etc. The use of the magnetometer instead of a dynamic head greatly reduces control and synchronization problems in the device, since with a magnetometer, an unknown bill may be examined and correlated to a known counterpart in space without reference to time whereas the use of a dynamic head may require accurate correlation with space and time.

It is another object of the invention to provide such apparatus in which, at each instant while the magnetized bill moves with respect to the magnetometer, the magnetometer measures magnetic flux from only a very small fragment of the pattern of indicia on the bill so that the magnetometer sees the pattern of indicia with maximum detail.

It is another object of the invention to provide an improved flux sensing head portion for the magnetometer used in apparatus of this type whereby the head portion will be able to see greater detail in a pattern of printed indicia than has been possible heretofore.

It is another object of the invention to provide apparatus for recognizing the validity and denomination of printed currency in which means are provided for magnetizing the ink on currency in a direction perpendicular to the face of the currency and then move the currency with respect to a magnetometer which will generate a detection signal which is a parameter of the pattern of indicia.

It is another object of the invention to provide such apparatus in which the magnetometer has extreme sensitivity both in its ability to detect minute quantities of magnetic flux and in its ability to detect such quantities of flux emanating from minute physical areas.

It is another object of the invention to provide such apparatus having means for adjusting the magnetometer output to a starting condition automatically prior to each time it is used for recognizing a piece of currency.

It is another object of the invention to provide such apparatus in which counterpart means are provided for synchronous comparison with the detection signal generated by the magnetometer so that an unknown bill may be recognized responsive to similarity between a predetermined reference and the detection signal derived from the unknown bill.

It is another object of this invention to provide such counterpart means containing a physical counterpart of a valid piece of currency in Whi the physical counterpart has the form of a shaped electrical conductor which may be prepared in a simple manner as a parameter of the signal which would be produced by the magnetometer when the valid piece of currency was examined thereby.

It is another object of the invention to provide particular systems for recognizing printed currency in this manner which do not employ sliding switch contacts whereby these particular systems may be employed in situations Where sliding switch contacts deteriorate through corrosion etc.

It is another object of the invention to provide apparatus for scanning pieces of U.S. currency and generating an electrical signal with the amplitude and polarity of the signal varying as a function of the printed pattern scanned by the apparatus and with the apparatus having sufficient sensitivity that the signal generated thereby from any given U.S. bill will differ from the signal generated by the apparatus from any US. bill of different denomina- 4 tion.

It is another object of the invention to provide such apparatus with which the signals generated from all valid bills of each denomination will be similar to each other with the signals from bills of different denomination forming a group of predetermined signals which differ from each other.

It is another object of this invention to provide a method for distinguishing between the electrical signals of this group so that the US. bills represented thereby may be recognized and distinguished from each other.

It is another object of the invention to provide such a method by which any one of a group of different variable amplitude and polarity electrical signals may be recognized to the exclusion of the remainder of signals of the group.

Other objects and advantages of the invention and the inventive subcombinations thereof will become apparent from the following detailed description of several specific devices which may be employed in the practice of the principles of the invention, these specific devices being indicative however of but a few of the many ways in which the principles of the invention may be employed.

The above enumerated and additional objects of the invention are obtained by the separate and joint action of the following subcombinations illustrated in greater detail hereinafter: mechanical support and transport apparatus, improved magnetic flux measuring heads, synthetic counterparts for maintaining standard references to which an unknown piece of printed matter may be compared, and a plurality of composite systems employing the same together with a variety of means for controlling and regulating the systems.

These systems and subcombinations are illustrated in the accompanying drawings in which:

FIGS. 1 to 6 are drawings of physical support and transport means employed in the invention:

FIG. 1 being a side elevation of such transport means, FIG. 2 being a fragmentary view taken along the plane and in the direction indicated at 2-2 in FIG. 1 and illustrating specifically the drive reversing means shown in FIG. 1, FIG. 3 being a side elevational view of the apparatus taken from the opposite side as FIG. 1, FIG. 4, being a vertical section of the apparatus taken along the plane 44 of FIGS. 1 and 3, FIG. 5 being a view in end elevation of the apparatus, and FIG. 6 being a longitudinal section of the apparatus taken along the plane 66 of FIG. 5;

FIGS. 7 to 9 are drawings of three alternative forms of improved magnetic flux measuring heads forming a part of the invention and forming an important new subcombination having substantial utility and unique features apart from its use With other components of the invention:

FIG. 7 being a perspective view of a preferred form of head, FIG. 8 being an end elevation of an alternative form of improved head, and FIG. 9 being an end elevation of still another alternative form of improved head;

FIG. 10 is a composite drawing and circuit diagram of a preferred system of the invention showing the apparatus of FIGS. 1 to 9 interconnected with electronic means for rendering the apparatus operative automatically and efficiently;

FIG. 11 shows, first in the upper portion, parts of four wave forms which might be obtained by recording the detection signal produced by the magneto-meter shown in FIG. 10 while valid bills of four different denominations were examined by the apparatus and showing, second in the lower portion, four different shapes which would be used for the switching conductors on the rotatable drum shown best in FIGS. 4, 5 and 10, when the four different denominations indicated in the upper part of the drawing were to be recognized. For simplicity of illustration and description, the curves and shapes are aligned in synchronization vertically of the drawing, and the shapes in the lower part of the drawing are illustrated schematically;

FIGS. 12 and 13 are composite schematic drawings and circuit diagrams of alternative systems of the inven tion in which a new variable capacitor is employed as a synthetic counterpart signal generator for generating a predetermined electrical signal against which the detection signal derived from an unknown bill is compared:

FIG. 12 showing a system arranged for adding together the detection signal and synthetic counterpart signal, and FIG. 13 showing a system for multiplying the signals together;

FIGS. 14 and 15 are drawings of the new variable capacitor employed in FIGS. 12 and 13 as a synthetic counterpart signal generator and which may be employed in many instances for generating any desired predetermined synthetic signal apart from the remainder of the elements of the system;

FIG. 14 showing a perspective view of the variable capacitor, and FIG. 15 showing a sectional view through the apparatus of FIG. 14 along the plane 15-15;

FIG. 16 is a schematic diagram of another alternative system of the invention in which two magnetic flux measuring heads are employed to examine the finelined background of the picture on a bill; and

FIG. 17 is a schematic diagram of another alternative system of the invention similar to the system but employing dynamic flux detection instead of static detection.

Mechanical support and transport apparatus Referring now in detail to the drawings and particularly to FIGS. 1-6, the apparatus illustrated therein for transporting pieces of printed currency to be examined comprises a frame 20 on which are mounted four axles 21, 22, 23, and 24, each of the axles carrying a roller with belts 26 and 27 extending over horizontally disposed pairs of rollers 25 as indicated. The belts 26 and 27 are preferably made of thin sheets of suitable plastics which contain no magnetizable substances.

The axle 23 (as best seen in FIG. 3) extends laterally from the frame 20 and may be attached to a suitable motor to be driven thereby, the axles 22 and 23 carrying gears 28 and 29 thereon by which the two belts 26 and 27 are driven in synchronization. A loading tray 30 (see FIG. 1) is mounted adjacent to the bite of the rollers 25 on axles 21 and 24 to deliver to the belts 26 and 27 pieces of printed currency inserted in the tray 30. A start switch 31 actuated by a photoelectric cell 32 is mounted adjacent to the tray 30 and positioned to be closed when a piece of currency is inserted in the tray 30 interrupting light transmission to the cell 32; the switch 31 is connected to the motor (not shown) which drives the axle 23 to start operation of the motor. For reasons that will appear hereinafter the width of the tray 30, measured in a plane perpendicular to FIG. 1, is preferably substantially equal to the width of pieces of printed currency which are to be examined by the apparatus whereby pieces of printed currency are fed to the belts 26 and 27 along a path generally parallel to the lengths of the pieces of currency.

A pair of horseshoe magnets 33 and 34 are mounted adjacent to opposite edges of the belts 26 and 27 rigidly attached to the frame 20, and a pair of magnetic pole pieces 35 and 36 are rigidly attached to like ends of the magnets 33 and 34 whereby one pole piece becomes a north pole and the other pole piece becomes a south pole with a magnetic field extending between the pole pieces 35 and 36 generally perpendicular to the belts 26 and 27. It should be noted that the belts 26 and 27 convey pieces of printed matter between the pole pieces 35 and 36 so that magnetizable particles in the ink with which the currency is printed are magnetized in a direction perpendicular to the faces of the piece of currency, one face of the piece of currency becoming a north pole and the other face becoming a south pole.

A pair of brackets 37 and 33 are rigidly attached to the frame 20 by dowels 39 and screws 40 with the brackets 37 and 38 being positioned on opposite sides of the adjacent courses of the belts 26 and 27 so that pieces of printed matter conveyed by the belts will pass between the brackets 37 and 38 subsequent to their passage between the magnetic pole pieces 35 and 36. Accordingly it should be noted that normal operation of the motor which drives axle 23 causes rotation of the rollers 25 and the gears 28 and 29 in the direction indicated by the arrows 41. As explained in greater detail hereafter, a magnetic flux measuring head portion of a magnetometer is mounted on the brackets 37 and 38 to measure the pattern of magnetic flux surrounding printed indicia on currency supported by the belts 26 and 27. The brackets 37 and 38 are mounted centrally of the width of belts 26 and 27 so that the head portion of the magnetometer will see the same strip of a bill regardless of which face of the bill is placed upwardly in the tray 30.

It will thus be noted that this apparatus provides means for maintaining the pole pieces 35 and 36 and the flux sensing head in brackets 37 and 38 in stationary positions fixed with respect to each other while a piece of currency to be examined is moved with respect to them. While it may be possible to reverse this procedure and move the pole pieces and flux sensing head while holding the piece of currency stationary, it has been found that the manner of operation illustrated is much more desirable since the reverse procedure necessitates excessive shielding of the flux sensing head, since when the head is moved it is moved through the earths magnetic field, and it has been found that the quantities of magnetic flux sensed by the head from pieces of currency are substantially less than the quantities of flux in the earth magnetic field through which the head would have to pass if it were moved.

As explained in connection with FIG. hereafter, passage of a piece of currency along the path between the pole pieces 35 and 36 and the brackets 37 and 38 produces a detection signal which is a parameter of the pattern of printed indicia on the currency. This detection signal is compared continuously with a physical counterpart derived from a pattern of printed indicia on a valid piece of currency of known denomination, favorable comparison effecting recognition of the test bill, and unfavorable comparison effecting rejection of the test bill. When the test bill is to be rejected, the direction of movement of the motor driving axle 23 is reversed to thereby deliver the test bill back to the end of the apparatus adjacent to the tray 33.

Synchronization of comparison between the detection signal and the physical counterpart of a known bill is effected by mounting the physical counterpart on a rotatable drum 42 (see FIGS. 26), rotating the drum 42 by means of the mechanical linkage shown in FIG. 2, and electronic synchronization means to be described in greater detail hereinafter. The drum 42 is rotatably supported on the frame 219 by means of a pair of stub shafts 43 and 44. The drum 42 is provided with an internal, generally conical, friction lining 45 engageable with a clutch surface 46 on a disc 47, with the surfaces and 46 being resiliently urged together by means of a compression spring 48 engaging an internal shoulder 49 in the drum 42. The drum 42 is freely rotatable on the shafts 43 and 44 and is caused to rotate solely by frictional contact between the surfaces 45 and 46. The disc 47 is rigidly attached to the shaft 44, and the shaft 44 carries a drive gear 59 rigidly attached thereto.

A shaft 51 is mounted on the frame 20 and carries an inner gear 52 (see FIG. 2) and an outer gear 53 with the gear 53 being engaged with the drive gear 50. A reversing shoe 54 is frictionally mounted on the shaft 23 by means of bolts 55 so that normal rotation of the shaft 23 urges counterclockwise rotation of the shoe 54, and reverse movement of the shaft 23 urges clockwise rotation of the shoe 54. The shaft 23 carries a gear 56 thereon, and the shoe 54 carries a forward drive gear 58 and a reverse drive gear 59 thereon with both of the gears 58 and 59 in permanent engagement with the gear 56. The gears 53 and 59 are arranged as indicated so that (1) the gear 58 engages with gear 52 when the shoe 54 rotates counterclockwise to thereby effect rotation of drive gear 50 in the direction of arrow 66, and (2) the gear 58 disengages from gear 52 and the gear 59 engages with gear 50 responsive to clockwise rotation of the shoe 54 to thereby rotate the drive gear 50 in the direction of arrow 60 regardless of the direction of rotation of the shaft 23. This constant directional rotation of the drive gear 50 and hence the drum 4?. regardless of the direction of rotation of the shaft 23 permits the apparatus to recognize a recognizable piece of printed currency regardless of the manner in which the piece of currency is inserted into the tray 39. As indicated above, the magnetic detection head in the brackets 37 and 38 is mounted to scan a longitudinal track along the center of a piece of currency between the belts 26 and 2'7 so that the head will see the same pattern of indicia regardless of which face of the bill is placed upwardly in the tray 39. Also, the bill can be reversed end for end in the tray 30 without preventing the apparatus from recognizing it since if the head sees the pattern of indicia backwards with reference to the counterpart on the drum 42 when a bill is moving into the machine (toward the left as viewed in FIG. 1), reversal of the shaft 23 to reject the bill will permit the head to see the pattern of indicia properly as the bill moves out of the machine (to the right as viewed in FIG. 1) suitable switching (not shown) being provided to permit the head to look at and recognize the bill while it is passing in both directions.

While synchronous drive is provided between the shaft 23 and the counterpart drum 42 through the gears of FIG. 2 and the clutch surfaces 45 and 46, this driven connection is electronically synchronized at the beginning of each test and is electronically re-synchronized a plurality of predetermined times while a piece of currency is passing the magnetic flux detecting head. This rte-synchronization permits the apparatus to recognize all bills of any given denomination even where different spacings are found between different local areas of printed indicia on the bills of a given denomination. This electronic synchronization is provided by the use of a plurality of mechanical stops for interrupting rotation of the rotatable drum 42 just prior to the beginning of each test and at a plurality of rotational positions of the drum during the passage of a piece of currency under the head. The mechanical stops are then released electronically responsive to the detection of magnetic flux by the head so that rotation of the counterpart drum 42 is initially synchronized and is later re-synchronized responsive to the actual arrival of areas of printed matter at the magnetic detection head.

As may be seen most clearly in FIGS. 4 and 6 and in a corresponding portion of the apparatus of FIG. 14, a plurality of stop members 61 are mounted on one end of the drum 42 by means of bolts 62, and the members 61 have stop ends 63 axially spaced of the drum from each other and cooperating with a corresponding plurality of adjustable stop screws 64 so that rotation of the drum 42 is interrupted when the stop end 63 of one of the stops 61 contacts its corresponding stop screw 64. The stop screws 64 are mounted on the movable arm 65 of a stop release relay 66 so that energization of the relay 66 pulls the stop screw 64 to the left as viewed in FIG. 6 to thereby release them from the stops 61. It should here be noted that the stop screws 64 are mounted on a spring arm 67 against the resiliency of which the Stop screws 64 are moved when they contact the stops 61. This movement of the stop screw 64 in opposition to spring 67 causes the opening of a pair of normally closed switch contacts 68 and the closing of a pair of normally open contacts 69. The functions of these switch contacts 68 and 69 will become more apparent in con ection with the description of FIG. 10.

In addition to the switches 68 and 69 which are actuated by the drum 42, a cam actuated stack of switches 70 (see FIGS. 1 and 10) is actuated by a roller 70a which moves along a track on the opposite edge of the drum 42 from the stops 61. The stack of switches 70 includes seven contacts 7177 and a pair of insulators 78 and 79. The contacts 71-73 define a ingle pole double throw switch in which the contacts 71 and 72 are closed at the starting rotational position of the drum (when the roller 70a descends into a notch 80 in the drum 42) (see FIG. 10). At all other rotational positions of the drum 42, the switch contacts 71 and 72 are open, and the switch contacts 72 and '73 are closed. A plurality of small areas 81 of the periphery of the drum 42 are provided with slight rises therein which effect closing of the switch contacts 74 and 75, and a large cam 82 is provided on the drum positioned to contact the roller 70a just before the roller 70a moves into the notch 80. When the roller 70a moves over the large cam 82, the switch terminals 76 and 77 are connected. As will appear from the description of FIG. 10, the switches 68 through 77 are employed to control the automatic logic which forms a portion of the apparatus.

Flux sensing head portion of magnetometer As mentioned heretofore an important feature of the invention is the provision of an improved magnetic flux sensing head which has sufficient sensitivity from the standpoint of measuring small quantities of flux and resolving closely spaced lines of printed matter that the apparatus of this invention is capable of recognizing pieces of currency and distinguishing between different denominations thereof by scanning the magnetic pattern of a magnetized bill. This sensitivity is obtained through the new design of a flux measuring head used in a fiux gate magnetometer, that is, the type of magnetometer in which measurement of magnetic flux is accomplished by periodically varying the reluctance of a magnetic conductor with a driving magnetic field.

Referring more specifically to FIG. 7, the fiux sensing head illustrated therein comprises a pair of blades 83 and 84, a magnetic shunt 85, and two pairs of magnetic conductors 86 and 87 connecting the shunt 85 to the blades 83 and 84 respectively. The elements 83-87 are preferably constructed of a very low magnetic reluctance material such as Mu-metal, and these elements define a magnetic conductor having opposite ends 88 and 89 which are beveled with the blades 83 and 84 slanting away from each other as they diverge from the edges 88 and 89. The flux sensing head shown in FIG. 7 is illustrated at several times its actual size, the edges 88 and 89 being very closely spaced together (on the order of I-30 thousandths of an inch apart), and this magnetic conductor is enclosed within a magnetic shield 90 having a shaped aperture 91 therein with the edges 88 and 89 of the magnetic conductor positioned in the aperture 91. A backing late 92 constructed of a relatively low reluctance material is mounted adjacent to the edges 88 and 89 of the magnetic conductor and the aperture 91 of the shield with the spacing between the backing plate and the shield and conductor being very small. In the operation of this head, magnetic flux flowing generally perpendicular to the backing plate 92 is measured as it flows through the magnetic conductor, the flow of magnetic flux through the conductor being caused by differences in magnetic potential adjacent to the two edges 88 and 89. Thus, assuming a higher potential adjacent the edge 88, flux would flow from backing plate 92 to the edge 88 upwardly through the strips 86, across the shunt 85, down the strips 87, through the edge 89 and back to the backing plate 92. Thus, it will be noted that measurable flux passes through the conductor only when a gradient of magnetic potential exists between the edges 88 and 89, and with these edges so close a very high degree of resolution is obtained with the head. To some extent magnetic flux is also caused in the magnetic conductor for instance by the flow of flux from the edges of the aperture 91 in the shield 90 to the edges 88 and 89 of the magnetic conductor, but such induced magnetic flow is very small when compared to the flow caused by differences in magnetic potential immediately adjacent to the edges 88 and 89. It should be noted that the positioning of the edges 88 and 89 within the aperture 91 of the shield 90, together with the provision of the backing plate 92, provides an extremely sensitive flux measuring device since the magnetic conductor is affected by very little flux other than the flux immediately adjacent to its edges.

The geometrical area from which magnetic fiux flows to the conductor can be further controlled, when desired by shaping the backing plate 92; the provision of a ridge on the backing plate 92 immediately under the aperture 91 and parallel to the aperture 91, and the provision of such ridges extending perpendicular to the aperture 91 may be desirable in some instances.

When magnetic flux flows along the above-mentioned path through the conductor, it is measured by electrical coils 93 and 94 which are Wound on the strips 86 and 87 respectively. The coils 93 and 94 are connected together as indicated so that alternating current input at terminals 95 will alternately establish closed magnetic loops in the conductor first in the direction of the arrows thereon and then in the opposite direction. The establishment of these closed loops of magnetic flux especially when the input current is sufficiently high to saturate the strips 86 and 87, effectively provides switches which turn on and oil. the flow of measurable flux through the conductor from the backing plate 92 as described above. When the input alternating current at terminals 95 has a predetermined frequency, the path of measurable flux is turned on and otf at an even multiple of the predetermined frequency, and as this measurable flux is turned on and off, it induces current in the coils 93 and 94. When detection leads 96 are connected to the coils as illustrated, this even harmonic induced current can be detected from these terminals. Thus, with this arrangement an input current at frequency F is applied to terminals 95 and an output signal is detected at terminal 96 at a frequency of 2F. The construction of the head as illustrated provides maximum sensitivity of the head first from the standpoint of producing large signals at terminals 96 compared to noise and secondly from the standpoint of the production of these large signals from very small magnetic sources in very small physical areas between the edges 88 and 89 of the conductor and the backing plate.

A similar magnetic head is shown in FIG. 8 where the flux gate coils 93 are provided in only one leg (83) of the magnetic conductor. A further alternative form of head is shown in FIG. 9 where the magnetic conductor is provided with only one leg and return path for magnetic flux flowing to the backing plate is provided through the shield 99 surrounding the magnetic con-ductor. In this form of the head, the path of measurable flux is indicated by the arrows 97, and the path of the closed loop flux which turns 1 1 the measurable flux on and oif is indicated by the arrows 98.

For some particular applications of the magnetometer heads of this invention particularly where they are used in currency recognizing apparatus, it may be desirable to employ devices sensitive to the Hall effect as the magnetic flux measuring element in the head. In such an application where the head of FIG. 8, for instance, is to be used, the conductor strips 86 and the coils 93 may be replace-d by a wafer of a material sensitive to the Hall effect with the plane of the wafer perpendicular to the path of magnetic flux in the conductor 83. An A.C. drive current of predetermined frequency is then applied through the wafer along the plane of the wafer, and a voltage proportional to the magnetic flux in the conductor 83 is detected across the water in a direction perpendicular to the direction of the drive current and having the same A.-". frequency as the drive current.

Synthetic counterpart known piece of currency As mentioned heretofore the apparatus of FIGS. 110 employs a physical counterpart which is a parameter of the detection signal which would be generated by the magnetometer when a piece of printed currency of known validity and denomination was delivered to the belts 26 and 27. In FIGS. 4 and 5, the drum 42 is illustrated as carrying such physical counterparts for three different denominations of printed currency. Thus, electrical switch wipers 107 are provided for contacting pairs of switch projections 108 and 109 with the pairs of projections carried respectively by electrical conductor strips 110 and 111 for the projections 108 and conductor strips 111 and 112 for the projections 109. These conductor strips and projections form a counterpart of a one dollar bill. Only those conductor strips and projections are illustrated on the drum 42 in FIG. 10 for the sake of simplicity of illustration, however, additional switch wipers are illustrated in FlG. for the other denominations, a pair of wipers 113 and associated strips being provided for recognition of five-dollar bills, and a pair of wipers 114 and associated strips being provided for recognition of ten-dollar bills. As explained in detail hereinafter, each of the pairs of wipers 107, 113 and 114 cooperates with three of the electrical conductor strips on the drum 42, the wipers 113 cooperating with the strips 112, 115 and 116, and the wipers 114 cooperating with the strips 116, 117 and 118. Preferably, a selector switch is connected to these three pairs of wipers 107, 113 and 114 to connect a selected pair to the comparison and logic parts of the device, so that one set of comparison and logic parts may recognize a plurality of denominations. For a totally automatic machine however, or for a similar currency sorting machine, a plurality of sets of comparison and logic parts are preferably used.

As illustrated in FIG. 11, the shapes of the conductor strips on the drum are chosen from a knowledge of the predetermined detection signals which would be generated by the magnetometer when pieces of printed currency of known validity and denomination were moved past the magnetometer head by the belts 26 and 27. \Vhile the shapes of the conductor strips are selected, they are parameters of these predetermined detection signals because the predetermined detection signals are first recorded and then employed as explained hereafter as the basis for proper selection of the shapes of the conductor strips. An auxiliary group of switch wipers is connected to the conductor strips as illustrated in FIGS. 4 and 5 with the auxiliary switch wipers 119 connected to the conductor strips 110, 112, 116, and 113, and auxiliary wipers 120 being connected to the conductor strips 111, 115, and 117. The wipers 11": are connected to one of the output leads of the magnetometer, and the wipers 120 are connected to the other output lead of the magnetometer so that at any given instant, the output of the magnetometer is applied to the conductor strips on the drum with one polarity of the output being applied to the wipers 119 and the other polarity to the wipers 120. Referring for simplicity to FIG. 10, and assuming that the wiper is either positive or negative with respect to the wipers 119 it will be seen that the polarity of the detection signal conducted to the pickup wipers 107 will be determined by the rotational position of the drum 42 since contact between wipers 107 and switch projections 108 will conduct one polarity of the detection signal, and contact between the wipers 107 and the switch projections 109 will conduct the detection signal to the Wipers 107 with opposite polarity.

It should be here mentioned that recognition of a piece of printed matter is accomplished with the apparatus of FIG. 10 by integrating the detection signal from the magnetometer over the period that a piece of currency passes the detection head. Since this magnetometer is, however, sensitive to the rate of change of the density of the pattern of printed indicia on the currency, the unaltered integral of the detection signal during the period a bi'tl is examined would give a zero integration sum. The function then of the counterpart switching elements on the drum 42 is to select known portions of the signal a bill is expected to give, reversing the polarities of these portions where desired, and integrating the detection signal after adjusting the polarity of its parts. In other words, when the wipers 119 are positive with respect to the wiper 120, and a valid one-dollar bill is being examined, the wipers 107 will con tact switch projections 108 yet when the polarity of the detection signal reverses so that wiper 120- is positive with respect to wiper 119, the wipers 107 will contact the switch projections 109.

It will thus be seen that the conductor strips 110, 111, and 112 on the drum 42 together with the switch wipers 107 form a physical counterpart of the detection signal which a valid one-dollar bill is known to give, this physical counterpart operating to permit the production of a high integration sum by integrating the signal between the wipers 107 if the polarity of the detection signal generated by the magnetometer varies in the same manner as the predetermined detection signal from which the shape of the conductor strips has been derived.

To further illustrate the manner of selection of the shape of the conductor strips on the drum 42, reference is made to FIG. 11 in which the upper four curves represent as indicated portions of the wave form of the detection signal which may be generated by a particular magnetometer when four pieces of currency of known validity and of different denominations are examined by the magnetometer. It should be noted that these four Wave forms may represent the signals from individual one, five, ten, and twenty dollar bills, but in fact that are at least three kinds of five dollar bills for instance in general circulation which produce three ditferent signals, and when a five dollar bill is to be recognized, all three of these signals should be recognized while rejecting the signals which would be produced by all kinds of bills of other denominations. Each wave form indicates the amplitude and polarity of each detection signal plotted against the position on the bill at which the amplitude is generated. The characters in the lower portion of the drawing show corresponding switching patterns which could be derived from the wave shapes for recognizing each indicated denomination of currency. While it is understood that three shaped conductors are employed for regulating the polarity of the detection signal for each denomination, the simple graphs are shown in indicating the use of the three conductor strips to effect regular polarity where a projection projects upwardly from the median line and reverse polarity when the projection extends downwardly. With this in mind consider the locations of projections 121, 122, and 121a in the pattern for the recognition of a one-dollar bill. It will b noted that an integrator connected between projections 108 for regular polarity at area 121 will accumulate a positive integral whether the bill being examined is a one, a ten or a twenty, but that if the bill being examined is a five the integrator will accumulate a negative sum thus discriminating against a five. At a later time then the integrator is connected between projections 108 for regular polarity at area 122, the integrator will again accumulate a positive sum for a one-dollar bill but will accumulate substantially no sum for the five and large negative sums for the ten and twenty. When the integrator is connected for reverse polarity at area 121a, the integrator will accumulate substantially nothing from a ten or twenty dollar bill; it will accumulate a large positive sum from the negative signal of the one-dollar bill, and it will accumulate a large negative sum from the positive signal of the five-dollar bill. Proper selection of the positioning and the direction of these projections will result in the recognition of any one denomination and the refusal to recognize bills of any other denomination.

This proper selection of the positioning and direction of the projections is accomplished as follows: The detection signals from each kind of bill of each denomination are recorded as time line graphs for instance during the period when the bills pass the head of the magnetometer; portions of this period are then selected during which the one-dollar signals, for instance, differ substantially from at least one of the remainder of the signals; the selected portions are then divided into two groups where (1) in the period portions of the first group, the one-dollar signal is substantially more positive than the other signal and (2) in the period portions of the second group, the onedollar signal is substantially more negative than the other signal the switching projections are then reversed for the two groups, the projections represented at 121 and 122 in FIG. 11 being in one group while the projections represented at 121a in FIG. 11 being in the other group.

Thus, regular polarity projections may be selected for recognizing the one-dollar bill where: (l) the one-dollar signal is strongly positive and any one of the remainder of the signals is strong negative, slightly negative, zero, or slightly positive, (2) the one-dollar signal is slightly positive and any one of the remainder of the signals is strongly negative, slightly negative or zero, (3) the onedollar signal is zero and any other signal is strongly or slightly negative or (4) the one-dollar signal is slightly negative and any other signal is strongly negative. Similarly, reverse polarity projections may be selected where: l) the one-dollar signal is strongly negative and any one of the remainder of the signals is strongly positive, slightly positive, zero, or slightly negative, etc. The positions and polarity of the projections in the period of the signals are preferably chosen where the signal to be recognized differs substantially from all other signals, but in many instances this is not possible and positions must be chosen where it is possible to discriminate against only a few or only one of the undesired signals.

Preferred complete system Referring now in detail to FIG. 10, the elements and subcornbinations described above are here illustrated in combination together with various electronic means for controlling the complete system. Here the magnetic detection head 100 shown in FIG. 7 together with its backing plate 101 are shown enclosed in a double magnetic shield 102 in the brackets 38 and 37, and a reversible drive motor 103 is shown driving the shaft 23 and the counterpart drum 42 through the friction clutch 47. The magnetic head is shown connected to a conventional head drive oscillator 104 which supplies the alternating input current to the leads 95 of the detection head. The magnetic head 100 is also connected to a conventional tuned amplifier and synchronous detector 105 by which the detection signal from terminals 96 of the head are amplified and converted to a detection signal delivered to leads 106 with the detection signal being a direct current signal the amplitude of which is proportional to the quantity of measurable flux flowing in the magnetic conductor of the head of FIG. 7 and the polarity of which at any instant is determined by 1 the direction measurable flux is flowing in that magnetic conductor. For the sake of convenience, the complete magnetometer which produces this detection signal is referred to as comprising a flux sensing head portion thereof which is mounted adjacent to the path between the belts 26 and 27 and a power means portion thereof which includes the head drive oscillator, tuned amplifier and synchronous detector which may be located either adjacent to the head or at a substantial distance therefrom.

As indicated above, the switch wipers 107 in FIG. 10 are connected to an integrator 123 which is arranged here to be set initially at a high voltage permitting the detection signal to integrate it downwardly during the period a bill is examined. The integrator is initially set at a fairly high voltage and connected to the switch wipers 107, as explained hereafter, so that a signal which is to be recognized will integrate the integrator sum downwardly to a lower voltage while undesired signals will integrate the sum upwardly to a higher voltage. The integrator is preferably provided with latching means for latching the integrator sum at a high voltage at any time that the voltage increases more than a predetermined amount from the voltage initially set. Such latching means may be provided by a Zener diode connected to turn on a tube when the integrator voltage exceeds the switching voltage of the diode with the tube being connected to the integrator to raise the integration sum additionally, when the tube is turned on.

The voltage accumulated by the integrator, which represents the integral of the signal conducted to the integrator, is conducted to a voltage comparator 124 by lines 125. The comparator is arranged to compare the voltage across lines 125 to a preset low voltage and conduct current to leads 126 when the signal from wipers 107 has integrated the voltage in leads 125 down to the preset voltage of the comparator. This current in leads 126 from the comparator is conducted to recognition logic 130 through the coil of a normally open relay 131 to close the relay 131 responsive to current in leads 126. It should be noted that the switch terminals 68 on the stop actuated switch of FIGS. 5 and 6 are connected in parallel with the switch terminals of relay 131 so that either the relay 131 or the switch 68 when closed can maintain a normally open relay 133 in logic 13%) energized through a pair of contacts 134 of relay 133. At all times that relay 133 is energized, it maintains a pair of its contacts 135 closed permitting a relay 136 to be energized therethrough when switch terminals 76 and 77 of the cam operated switch 70 close just prior to termination of a revolution of the drum 42. This relay 136 has a pair of switch terminals 137 thereof the closing of which is the act by which the logic circuit 130 recognizes a bill which has been tested by the apparatus. According, it should be noted that at any time when the drum 42 is moving and the switch contacts 68 are thus closed, the switch contacts 68 will maintain the relay 133 energized to thereby permit subsequent actuation of the relay 136 at the end of a cycle. When one of the stop members 61 contacts a stop screw 64 thereby opening the switch contact 68, the relay 133 will be de-energized un' less the comparator 124 has energized the relay 131 indicating that the preceding area of the bill being examined had accumulated a sufficient integral to actuate the comparator.

If at any time the drum 42 is stopped and the switch 68 opened, when the integrator 123 has not yet accumulated a suflicient integral to actuate the comparator, the relay 133 will be tie-energized because both the switch 68 and the relay 131 are open, and the relay 133 will remain de-energized until the drum 42 returns to its starting position and switch contacts 72 and 71 are closed; note that this is the only way relay 133 can be reenergized since its own contacts 134 are now open. As soon as relay 133 is tie-energized, a relay 139 in the logic 130 will be energized through contact 140 of the relay 131, and relay 139 has a pair of normally open contacts 141 the closing of 15 which is the act by which the logic circuit 138 rejects a test bill.

It should be noted that the provision of the latching means with the integrator adds substantial flexibility to the above-mentioned method of selecting the positions and directions of the switch projections on the drum 42. Thus any particular denomination bill may be rejected by positioning the projections so that the first part of its signal which is conducted to the integrator causes the integrator to latch at high voltage.

It should be recalled that a pair of switch contacts 74 and 75 on the cam actuated switch 70 are closed at desired portions of the period of rotation of the drum 42, and it should be noted that these switch contacts close a circuit to the tuned amplifier 105 whereby the gain of the amplifier is increased until the contacts are again opened. This permits selective emphasis of local portions of the detection signal produced by the magnetometer.

indicated centrally of FIG. 10 is a multi-terminal relay 14-2 having an energizing coil 143 and a plurality of arms 144148. In the relay 142 the relay arms move to their arms upward position when the coil 143 thereof is not energized. It will be noted that the coil 143 of relay 142 is connected to be energized through the normally open contacts 69 of the stop actuated switch shown in FIGS. and 6 with the result that the relay coil 143 is energized and moves the arms 144-148 to their arms downward position every time that the drum 42 comes to rest. When the drum is moving and the arms are in their upward position, the arms 144 and 145 conduct the detection signal from the magnetometer to the switch wipers 119 and 120 on the counterpart drum 42; at the same time the switch arm 146 is inactive and the arms 147 and 143 conduct the polarity modified signal from switch wipers 167 on the drum 42 to the integrator 123.

When the drum comes to rest and the switch contacts 69 are closed, the arms 147 and 145 are connected to a voltage divider to reset the initial level of the integrator so that it can start to integrate downwardly when the drum begins to move again; at the same time switch arm 146 is connected to a source of positive voltage in connection with a timing capacitor 149 to prevent the resetting of the integrator from tie-energizing the relay 133. Thus, high voltage is conducted to the comparator 124 with a time delay determined by the capacitor 149 to insure that the comparator energizes the relay 131 after the time delay. During the time delay the relay 131 may be energized or de-energized depending upon the prior condition of the comparator and integrator, and the period of the time delay is selected to be just shorter than the time necessary for the resetting of the integrator to influence the comparator. Also when the coil 143 is energized the arms 144 and 145 thereof move to their downward position to conduct the next portion of the detection signal from the magnetometer to a thyratron 156 which is in turn connected to the stop release relay 66. Generation of the next portion of the detection signal from the magnetometer will thereby actuate the stop release relay 66 and restart motion of the counterpart drum 42 in synchronization with motion of a piece of currency along its path under the magnetometer head.

It should also be noted in FIG. 10 that an additional actuating circuit is provided for the thyratron 1511, this circuit being connected through switch contacts 72 and 73 and having a fairly long time constant, the purpose of the circuit being merely to actuate the thyratron and return the counterpart drum to its starting position in case of bill failure to pass, when the drum 42 stops for an unduly long time at some intermediate point in the cycle.

Finally it should be noted in FIG. 10 that a triode 151 has its plate and cathode connected to supply negative feedback to the direction terminals 96 of the magnetometer head. The grid of triode 151 is connected to the output of the power means portion of the magnetometer through a switch 152 to increase the amount of negative feedback to the head portion responsive to increases in the magnitude of the direct current detection signal, whereby when the switch 152 is closed the triode 151 automatically attenuates to a large degree the production of any detection signal by the magnetometer. The switch 152 connecting the grid of the triode 151 to the output of the magnetometer is a normally closed switch which is opened responsive to closing of the start 31 adjacent to the input tray of the device (see FIG. 1), and triode 151 is selected to have a very long leakage time for its grid potential so that the triode 151 will maintain a substantially constant level of negative feedback for a substantial period of time after operation of the apparatus is started responsive to closing of the switch 31 and the opening of switch 152. In order to increase the leakage time of the grid potential of triode 151, a capacitor 151a is connected to the grid between the grid and the switch 152. It should also be noted that means are provided for holding the switch 331 closed and the switch 152 open until the closing of switch contacts 71 and 72; this insures that the switch 31 will not stop operation of the drive motor before completion of a cycle and it assures that the switch 152 will not provide automatic suppression of the detection signal when a detection signal is desired. It should be noted further that a substantial advantage is provided through the use of this means for automatically attenuating the detection signal produced by the magnetometer particularly since the magnetometer employed in this invention and forming a part thereof is so highly sensitive. Small changes in the magnetic field around the device could produce an excessively biased detection head through which magnetic flux flowed continually thereby producing a detection signal which was not the desired parameter of the pattern of printed indicia on pieces of currency being examined by the device.

Alternative systems ofiering even greater sensitivity The systems illustrated in FIGS. 12 and 13 diifer from the system of FIG. 10 in that the systems of FIGS. 12 and 13 employ a synthetic counterpart signal generator (as illustrated in FIGS. 14 and 15) through which an alternating current signal may be passed to produce an output which is a parameter of the shape of a predetermined wave. Where the shape of the predetermined wave is identical to a graphical recording of the detection signal generated by the magnetometer plotted against time as a piece of currency of known validity and denomination passes the head portion of the magnetometer, the output of the synthetic counterpart signal generator may be identical to the original detection signal which was generated by the known piece of currency. This is substantially the arrangement which is shown in FIG. 12 other components of the apparatus of FIG. 12 being common to or similar to FIG. 10. In FIG. 12, the synthetic counterpart signal generator is indicated by numeral 153 the generator having a wave form 154 to be explained in greater detail hereinafter. An electrical conductor 155 is moved longitudinally of the synthetic counterpart 153 and in synchronization with movement of a piece of printed currency past the head portion of the magnetometer. A drive signal is conducted to the synthetic counterpart 153 via two leads 156 and 157 with the signals conducted to the two leads being out of phase from each other and having a frequency equal to the frequency of the signal picked up at contacts 96 of the head portion of the magnetometer. This arrangement produces an output signal from the conductor 155 where the signal is substantially identical to the signal produced at terminals 96 of the head portion of the magnetometer when the known piece of printed currency from which the wave form 154 was derived was past under the head portion. This output signal is added to the signal delivered to terminals 96 of the head portion of the magnetometer with the two signals 180 out of phase, 

